Today, most wallpapers arrive trimmed from the factory. But you can still find wallpapers that need trimming. They are usually high-end papers, historic papers, hand printed papers, and some grass cloth and vinyls. Back in the day, manufacturers added the excess to leave the seam matching to the paperhanger — a hangover from the time when wallpaper was more intimately connected to the textile industry. They would leave salvage and extra material for tailors to use. The additional edge would also help prevent damage while shipping.
Trim with a razor blade, not scissors. 
Hand printed papers need to be hand trimmed. 
Can you find the 2 vertical seams in the middle of this paper?
